Miniature resonating marker assembly

ABSTRACT

A miniature resonating marker assembly that includes, in one embodiment, a ferromagnetic core, a wire coil disposed around the core, and a capacitor connected to the wire coil adjacent to the magnetic core. The core, coil, and capacitor form a signal element that, when energized, generates a magnetic field at a selected resonant frequency. The magnetic field has a magnetic center point positioned along at least one axis of the signal element. An inert encapsulation member encapsulates the signal element therein and defines a geometric shape of the resonating marker assembly. The geometric shape has a geometric center point substantially coincident with the magnetic center point along at least a first axis of the signal element. The shape and configuration of the assembly also provides for a miniature signal element specifically tuned to resonate at a selected frequency with a high quality factor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.09/954,700, filed Sep. 14, 2001, which issued on Nov. 14, 2006 as U.S.Pat. No. 7,135,978.

TECHNICAL FIELD

This invention relates to locating devices, and more particularly tominiature resonating marker assemblies and methods of tuning the same.

BACKGROUND OF THE INVENTION

Medical procedures often require locating and treating areas within apatient's body. Imaging systems, including x-ray, MRI, CT, andultrasound have been used to help locate areas or particular targetswithin the body. While the imaging systems can be very useful in somesituations, they can be limited to two dimensional information and maybe unusable or difficult to use in certain procedures to provide realtime three dimensional location information about a target.

Many noninvasive medical procedures, such as radiation therapy andsurgical procedures, require precise location information about thetarget to minimize the extent of collateral damage to healthy tissuearound the target. Markers have been used to locate targets on and in apatient's body in preparation for a medical procedure. One exampleincludes the use of gold fiducials, which are solid, inert, metal beadsthat can be implanted in a patient at or near a tumor or other targetthat may be difficult to accurately detect using conventional imagingsystems. The fiducial markers are passive markers that are easy todetect with imaging systems such as x-ray or ultrasound systems, but thepassive markers do not provide active real-time location informationduring a medical procedure.

Active, implantable marker assemblies that generate a detectable signalhave been used to locate a selected target or the like in real time.Many of the active markers are implantable in a patient, but they arehard-wired to a power source or other equipment external from thepatient. These hard-wired markers are removed from the patient's bodyafter a procedure or a series of procedures are concluded. Thehard-wired markers are often fairly large in order to provide desiredsignal strength, clarity, or other performance characteristics neededduring the procedure. The patient's body can typically temporarilyhandle the larger marker during the medical procedure before the markermust be removed. The removal process, however, requires an additionalinvasive procedure to the patient's body.

Leadless active markers also referred to as “wireless” active markers,have been developed to be implanted in a patient's body at or near aselected target, such as a tumor. The wireless active markers aretypically activated or energized to generate a detectable signal used tolocate the marker in the patient's body. Some wireless markers contain apower source, such as a battery, that provides the power to generate asignal detectable from outside the patient's body. The battery-poweredmarkers, however, typically must be removed after the medical procedurebecause the caustic materials in the battery are not suitable to leavein a patient.

The conventional, wireless active markers are also often fairly large inorder to provide a range of operating characteristics that allow themarker to be accurately located within the patient's body. Wirelessactive markers have experienced a trade-off between physical size andsignal strength. Larger active markers have been needed to provide therequired signal strength for detection and must be tuned adequatelyenough so that the detection system can detect the marker's signal. Thelarge active markers, however, have drawbacks, including a reducedaccuracy of determining the marker's precise location relative to atarget, the degree of invasiveness needed to implant the markers in thebody, and the costs of producing accurately tuned markers.

SUMMARY OF THE INVENTION

Under one aspect of the invention, a miniature resonating markerassembly is provided that overcomes drawbacks experienced in the priorart. In one embodiment, the resonating marker assembly includes a core,a wire coil composed of insulated wire disposed around the core, and acapacitor connected to the wire coil adjacent to the core. The core,coil, and capacitor form a signal element that, when energized,generates a magnetic field at the predetermined resonant frequency. Themagnetic field has a magnetic center point positioned along at least oneaxis of the signal element. An inert encapsulation member encapsulatesthe signal element therein and defines a geometric shape of theresonating marker assembly. The geometric shape has a geometric centerpoint substantially coincident with the magnetic center point along atleast a first axis of the signal element. Accordingly, when a userlocates the marker assembly's magnetic center point, the user will havealso located the marker assembly's geometric center point. Conversely,when a user locates the marker assembly's geometric center, the userwill have also located the marker assembly's magnetic center point.

In another embodiment, the miniature resonating marker assembly has aferromagnetic core with an elongated central portion and two enlargedend portions attached to or integrally connected to the central portion.A first end portion has an axial thickness different from the thicknessof the second end portion to define a core asymmetric about at least oneaxis through the marker assembly. A wire coil is disposed around thecentral portion of the core between the first and second enlarged endportions. A capacitor is connected to the coil adjacent to the core toform a signal element tuned to a selected resonant frequency. Themagnetic center point of the signal element is substantially coincidentwith the geometric center point of the resonating marker assembly.

In another embodiment, the miniature resonating marker assembly has acore made of a material having a relative permeability greater than 1.The core has an elongated central portion and two endcaps connected tothe central portion. One or more of the endcaps is axially movablerelative to the central portion. A wire coil is disposed around thecentral portion of the core between the first and second endcaps. Acapacitor, positioned adjacent to the core, is connected to the wirecoil. Test equipment is attached and the resonant frequency is measured.The first endcap is movable relative to the coil and the core's centralportion and is used to tune the circuit. Once the movable endcap ispositioned at the desired location, the endcap is secured in place, thetest equipment is removed from the circuit, and the signal element iscompleted by attaching the inductor lead to the capacitor terminal.

In another embodiment, the miniature resonating marker assembly has anelongated plastic sleeve with a wire coil disposed on the sleeve. Acentral portion of a core is placed within the sleeve, and a pair ofendcaps are connected to the sleeve so that the coil is positionedbetween the endcaps. A capacitor is operatively connected to the wirecoil and positioned adjacent to the core to form a signal element. Thecentral portion of the core is axially movable relative to the sleeveand the coil and is used for tuning the marker assembly to a selectedresonant frequency prior to completing the marker assembly.

In another embodiment, the miniature resonating marker assembly has acore with an end portion having a recess formed therein. A capacitor ispositioned in the recess, and a wire coil is disposed around the coreadjacent to the end portion and is operatively connected to thecapacitor to form a signal element tuned to resonate at a selectedfrequency.

In another embodiment, the miniature resonating marker assembly has acore with a central portion and a pair of enlarged endcaps connected tothe central portion. The central portion is made of a first materialwith a first magnetic permeability, and the endcaps are made of at leasta second material with a second magnetic permeability different than thefirst magnetic permeability. A wire coil is disposed around the corebetween the endcaps, and a capacitor is operatively connected to thecoil to form a tuned signal element.

In another embodiment, the resonating marker assembly has anannular-shaped capacitor with a central aperture, and an elongatedferromagnetic core extends through the aperture in the capacitor. A wirecoil is disposed about the core and is connected to the capacitor. Thewire coil has first and second portions. The first portion is disposedaround the core on one side of the capacitor, and the second portion isdisposed around the core on the other side of the capacitor.

In another embodiment, a method is provided for actively tuning theresonating signal element of a miniature marker during the manufacturingprocess. The method includes wrapping a first number of windings of anelongated wire around a central portion of a ferromagnetic core betweena pair of endcaps to form a wire coil with the first number of windings.The coil with the first number of windings and the core form an inductorwith a first inductance value. The first inductance value is activelymeasured and compared with a target inductance value. The number ofwindings forming the coil is then adjusted by adding or removing one ormore windings from the core to form a coil With a second number ofwindings to form an inductor with a second inductance valuesubstantially equal to the target inductance value.

In another embodiment, a method is provided for tuning a miniatureresonating marker assembly to have a target resonant frequency byplacing a core within a wire coil that has a plurality of windings. Thecore is made of a material with a relative permeability greater than 1.Lead lines of the wire coil are connected to a capacitor adjacent to thecore to form a resonating marker unit. Test equipment is attached formeasuring the resonant frequency. A first resonant frequency value ofthe resonating marker unit is measured and compared to the targetresonant frequency value. The core is moved axially relative to the wirecoil to adjust the resonant frequency value of the resonating markerunit to a second resonant frequency value substantially equal to thetarget resonant frequency value. The core is secured in a fixed locationrelative to the coil and the capacitor after the target resonantfrequency value is reached. After retesting to confirm that the desiredresonant frequency has been achieved, the test equipment is removed.(Lead lines have already been connected as stated above.) Once thecircuit has been completed, neither the inductance nor the capacitanceof the individual components can be directly measured. However, theimpedance and phase angle of the tuned circuit can be measured whichallows the resonant frequency and the quality factor (Q factor) of thetuned circuit to be determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of an implantable miniature resonatingmarker assembly in accordance with one embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of the marker assembly takensubstantially along line 2-2 of FIG. 1.

FIG. 3 is an enlarged cross-sectional view of a plurality of windings onthe marker assembly of FIG. 1.

FIG. 4 is an isometric view of a tunable resonating marker assembly inaccordance with an alternate embodiment of the present invention, anendcap of the assembly shown in a first adjusted position for tuning themarker assembly.

FIG. 5 is an isometric view of the resonating marker assembly of FIG. 4with an endcap in a second adjusted position for tuning the markerassembly.

FIG. 6 is an exploded isometric view of a miniature resonating markerassembly in accordance with an alternate embodiment of the presentinvention.

FIG. 7 is a cross-sectional view taken substantially along line 7-7 ofFIG. 6 showing a miniature inductor of the marker assembly with aferromagnetic core in an off-set position.

FIG. 8 is a cross-sectional view of a miniature resonating markerassembly in accordance with an alternate embodiment, with a two-pieceferromagnetic core.

FIG. 9 is an isometric view of a miniature resonating marker assembly inaccordance with an alternate embodiment utilizing a ferromagnetic pastebetween a ferromagnetic core and the wire coil. For the purposes ofclarity, the second ferromagnetic endcap is omitted from this figure.

FIG. 10 is an isometric view of a ferromagnetic core of a miniatureresonating marker assembly in accordance with an alternate embodiment,the core having a recess that receives a capacitor (not shown).

FIG. 11 is an enlarged isometric view of an endcap of the core of FIG.10 with the capacitor shown positioned within the recess.

FIG. 12 is an enlarged isometric view of an endcap of an alternateembodiment of the marker assembly of FIG. 10.

FIG. 13 is an enlarged isometric view of an endcap of an alternateembodiment of the marker assembly of FIG. 10.

FIG. 14 is an isometric view of a miniature resonating marker assemblyin accordance with an alternate embodiment of the present invention, anannular-shaped capacitor being shown positioned on the core between twosegments of the wire coil.

FIG. 15 is an isometric view of a miniature resonating marker assemblyin accordance with an alternate embodiment, wherein the ferromagneticcore has an I-beam cross-sectional shape.

FIG. 16 is an enlarged cross-sectional view of the marker assembly takensubstantially along line 16-16 of FIG. 15.

FIG. 17 is an enlarged cross-sectional view of an alternate embodimentof the marker assembly of FIG. 15.

FIG. 18 is a cross-sectional view of a miniature resonating markerassembly in accordance with an alternate embodiment with a plurality ofmodular endcaps positioned on a central portion of a ferromagnetic core.

FIG. 19 is an enlarged isometric view illustrating a process of windinga miniature resonating marker assembly in accordance with an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of theinvention. However, one skilled in the art will understand that theinvention may be practiced without these details. In other instances,well-known structures associated with resonating markers and activatorshave not been shown or described in detail to avoid unnecessarilyobscuring the description of the embodiments of the invention.

FIG. 1 is an isometric view of an implantable miniature resonatingmarker assembly 10 in accordance with one embodiment of the presentinvention. The marker assembly 10 is an inert, activatable assembly thatcan be excited to generate a signal at a resonant frequency detectableby a marker detection system external to the patient. An example of themarker detection system is described in detail in copending U.S. patentapplication Ser. No. 09/877,498, entitled Guided Radiation TherapySystem, which is incorporated herein in its entirety by referencethereto.

The marker assembly 10 includes a coil 12 wound around a ferromagneticcore 14 to form an inductor. The inductor is connected to a capacitor16, so as to form a signal element 18. Accordingly, the signal element18 is an inductor (L) capacitor (C) series circuit. The signal element18 is enclosed and sealed in an encapsulation member 20 made of plastic,glass, or other inert material. Accordingly, the marker assembly 10 is afully contained and inert unit that can be used, as an example, inmedical procedures in which the marker assembly is secured on and/orimplanted in a patient's body.

The marker assembly 10 can be activated by an external excitationsource(s) 22, so the signal element 18 generates a detectable signalthat allows the marker assembly to be precisely located, for example,during a medical procedure. The excitation source 22, in one embodiment,generates a magnetic field 24 at a selected frequency that substantiallymatches the resonant frequency of the specifically tuned marker assembly10. When the marker assembly 10 is excited by the magnetic field 24, thesignal element 18 generates a response signal at the resonant frequency90 degrees out of phase with the magnetic field. The marker assembly 10is constructed, as discussed in detail below, to provide anappropriately “loud” and distinct signal by optimizing markercharacteristics, such as the quality factor, for example, and byproviding an accurate and precise means of tuning the marker to apredetermined frequency to allow reliable detection by the markerdetection system 11. The signal from the accurately tuned markerassembly 10 is sufficient to allow the marker detection system 11 todetermine the marker assembly's identity, precise location, andorientation in three dimensional space.

The miniature marker assemblies 10 discussed below are accurately tunedto a chosen resonant frequency. Because the miniature marker assemblies10 are constructed to be very small, the markers must be tuned so that,when energized, they will provide a response signal that is strong andclearly distinguishable from the excitation signal, signals from othermarkers, and environmental noise. Accordingly, the signal will have astrength, clarity, and uniqueness that can be detected and analyzed bythe sensor system 26 to determine the precise location of the markerassemblies 10 on and/or within the patient relative to the sensorsystem. The information regarding the precise location and orientationof the marker assemblies 10 and the target areas is then usable to helpminimize collateral damage to healthy tissues around the targets duringradiation therapy, surgical procedures, or other selected medicalprocedures that require locating and tracking a specific tissue or areafor monitoring or treatment purposes.

FIG. 2 is a cross-sectional view of the miniature resonating markerassembly 10 of FIG. 1. The marker assembly 10 of one embodiment has agenerally cylindrical shape with an axial dimension of approximately2-14 mm, and a diameter of approximately 0.5-3.0 mm, inclusive. In otherembodiments, the marker assembly 10 can have other dimensions and theabove range of dimensions is provided as an illustrative example of thesize of one embodiment of the marker assembly. The marker assembly 10has the wire coil 12 formed from an elongated insulated copper wiretightly wound around the core 14.

In the illustrated embodiment, the core 14 is a material having arelative permeability greater than 1, such as a ferromagnetic material.In one embodiment, the core 14 is made from a selected ferromagneticmaterial, such as Fair Rite 78. The core 14 includes an elongatedcentral portion 28 and a pair of enlarged ferromagnetic endcaps 32attached to the ends of the central portion 28. The endcaps 32 in theillustrated embodiment are ferromagnetic endcaps. The wire coil 12 iswound around the central portion 28 between the endcaps 32. In theillustrated embodiment, the endcaps 32 are substantially cylindrical andeach has an outer diameter approximately the same as the outer diameterof the coil 12.

The wire coil 12 in the illustrated embodiment is made up ofapproximately 100-3,000 windings 30 of a low resistance, small diameter,insulated wire (e.g., 45-54 AWG, American Wire Gauge) tightly woundaround the core's central portion 28. The number of windings 30 in thecoil 12 depends upon the wire type, the wire size, the wire shape, thenumber of wires, the wind geometry, the core size, the core's material,the core's geometry, and the inductance required to tune the signalelement 18. As shown in FIG. 3, the coil 12 is formed by a large numberof windings 30 wound onto the core's central portion 28 so that eachwinding is immediately adjacent to all of its nearest neighbors. Forwire with a round cross-section as shown in FIG. 3, each winding, thus,will be immediately adjacent to up to six nearest neighbors.Accordingly, each winding 30 is nested in the valley area 31 formedbetween adjacent windings to provide a tightly packed windingconfiguration with the maximum number of windings within the volumebetween the endcaps. The tightly packed coil 12 on the ferromagneticcore's central portion 28 allows for a high inductance value to beachieved for the miniature marker assembly's small volume.

Alternatively, especially at higher frequencies, it may be desirable toincrease the distance between the coiled wires in a way that does notresult in random winding. One way to increase the distance in apredictable and repeatable manner is to use insulated wire having athicker insulation coating around the wire filament. Accordingly, thewire filaments in adjacent winds are spaced further apart, even thoughthe insulated wire is wound in the tightly packed configuration shown inFIG. 3. Another embodiment can use two (or more) parallel strands ofwire wound to form the coil, wherein only one of the wires, an activewire, is connected to the capacitor. The other wire is a spacer wirethat forms a spacer wind between each wind of the active wire.Therefore, the separation distance between consecutive winds of theactive wire is increased.

Another way to increase the distance between adjacent winds in anorganized but less tightly packed configuration is to place each wind ontop of adjacent windings rather than in the valley area 31. In thisconfiguration the coil is wound such that each wire is adjacent to onlyfour of its nearest neighbors.

As best seen in FIGS. 1 and 2, the elongated cylindrical marker assembly10 of the illustrated embodiment has a geometric center point 34. Thegeometric center point 34 can be determined by locating the midpointalong each of the marker assembly's length, width, and depth. A user candetermine the geometric center point 34 of an implanted marker assembly10 if needed by conventional techniques such as taking an image of theassembly with an x-ray or ultrasound device and physically measuring theimage of the assembly. The spatial relationship of the geometric centerpoint 34 relative to the target or other marker assemblies 10 may alsobe visually identified. When the marker is viewed using commonly usedmodalities such as x-ray, it is functioning similar to a surgical clipor other devices used to mark a specific tissue or region.

The signal element 18 in the marker assembly 10 also generates amagnetic field when the signal element is excited, and the magneticfield has a magnetic center point 36. If the core 14 was a symmetricalmember about the X, Y, and Z axis, the endcaps 32 would be the samesize, and the magnetic center point 36 would be offset from the markerassembly's geometric center point 34. The core 14 of the illustratedembodiment, however, is an asymmetric core with endcaps 32 havingdifferent thicknesses. The core 14 is shaped and sized so that themagnetic center 36 of the signal element 18 is coincident with thegeometric center 34 of the marker assembly 10.

The asymmetric configuration of the core 14 effectively shifts thecenter of the magnetic field axially along the length of the core. Thesignal element 18 can be configured and positioned in the encapsulationmember 20 so that the geometric and magnetic centers 34 and 36 arecoincident with each other. The coincident orientation of the geometricand magnetic centers 34 and 36 allows a physician or technician tononvisually determine the precise location of an implanted markerassembly 10 relative to a target during a medical procedure.

As an example, the marker assembly 10 can be implanted eitherpermanently or short-term in a patient and located visually with animaging system to determine the marker assembly's position and locationrelative to the target before initiating a selected medical procedure.The physician or technician can visually determine the marker assembly'sgeometric center 34 relative to the target. The information about thegeometric center 34 relative to the target can be utilized to providepatient set-up procedures or a treatment plan. The physician ortechnician will know that, when the marker assembly 10 is excited viathe excitation source(s) 22 (FIG. 1) and the location of the magneticcenter 36 is nonvisually determined in three-dimensional space, themagnetic center is at the same location as the geometric center 34 andhas the same relative orientation to the target. Accordingly, the markerassembly 10 can provide extremely accurate nonvisual informationregarding the marker assembly's actual real time location within thepatient's body relative to the target. That location information can beused to minimize the margins needed around the target when performing amedical procedure. If the geometric center point 34 and the magneticcenter point 36 are displaced by even small amounts, the margins aroundthe target may need to be larger, thereby potentially having a greaterimpact on healthy tissue around the target.

The asymmetric core 14 illustrated in FIG. 2 is shown as having agenerally cylindrical shape with the rod-shaped central portion 28 anddisk-shaped endcaps 32. In alternate embodiments, the endcaps 32 canhave other shapes, such as arcuate or semispherical shapes, that help toachieve the required characteristics of the tuned marker assembly 10.

For marker assemblies that will be implanted, it is highly desirable tominimize the size of the marker assembly 10 to allow for accurate andminimally invasive placement of the assembly into body tissues andcavities via such methods as introducer needles, endoscopes, catheters,etc. There are trade-offs, however, between marker size and signalstrength, because smaller markers typically provide weaker signals.Therefore, the miniature marker assembly 10 should be as small aspossible while still providing an adequate response signal overenvironmental noise when energized by the excitation source(s) 22. Tomaximize the signal strength of the marker assembly 10, the outerdiameter of the coil 12 and ferromagnetic core 14 should be maximizedrelative to the outer dimension of the marker assembly. The markerassembly 10 of the illustrated embodiment accomplishes this, in part,because the capacitor 16 and any other components, such as a tissuefastener 29 (illustrated in phantom lines connected to the markerassembly), are axially aligned with the core 14 and positioned at theproximal or distal ends of the core. The size constraints for markersplaced on the body are less stringent than for implanted markers.

FIG. 4 is an isometric view of the tunable miniature marker assembly 10with an adjustable endcap 32 shown in a first position. FIG. 5 is anisometric view of the marker assembly 10 of FIG. 4 with the endcap 32 inan axially adjusted second position for tuning of the marker assembly.In one scenario, the miniature marker assembly 10 is accurately tuned toprovide the maximum signal strength at the resonant frequency with amaximum quality factor for the signal. A primary factor in tuning themarker assembly 10 is determining when the impedance phase shift at thefrequency of interest is equal to zero. The impedance phase shiftbecomes zero when the capacitor impedance and the inductor impedance arematched for a given frequency. Alternatively, the marker could be testedin a wireless fashion where the phase shift at the resonant frequency ofthe marker would be 90 degrees out of phase when measured at the sensorsystem 26 (FIG. 1). When the signal element 18 is accurately tuned sothe capacitor and inductor impedances are matched for a given frequency,the signal element 18 will resonate at that selected frequency.

In the illustrated embodiment of FIGS. 4 and 5, one of the endcaps 32has an aperture therein that receives the core's central portion 28. Theendcap 32 is axially movable on the core's central portion 28 relativeto the coil 12 when the signal element 18 is being tuned. When theadjustable endcap 32 is positioned immediately adjacent to and abuttingthe coil 12 (FIG. 4), the endcap is positioned to reduce the inductanceof the inductor, thereby increasing the resonant frequency of the markerassembly 10. When the adjustable endcap 32 is positioned at the end ofthe core's central portion 28 spaced apart from the coil 12, the endcapis positioned to maximize the inductance of the inductor therebydecreasing the resonant frequency of the marker assembly. Accordingly,axial movement of the endcap 32 relative to the coil 12 can increase ordecrease the resonant frequency to fine tune the signal element 18.

During the manufacturing process of the marker assembly 10, the endcap32 is axially adjusted to precisely tune the marker assembly. In oneembodiment, the entire signal element 18 is assembled with the exceptionof the movable endcap 32. The movable endcap 32 is then slipped onto theend of the core's central portion 28 and positioned axially toward thecoil 12 to achieve the required inductance to obtain the desiredresonant frequency.

The axial position of the ferromagnetic endcap 32 can be determined byaxially moving the endcap 32 and simultaneously measuring the impedancephase shift at the frequency of interest with an impedance analyzer. Theimpedance analyzer is connected to the coil's lead wires that connect tothe capacitor 16. When the impedance analyzer determines that the phaseshift is equal to zero, the capacitor impedance and the inductorimpedance are matched for the selected frequency. The endcap 32 is thensecured in place on the core's central portion 28 with an adhesive, suchas a UV-cured or heat-cured epoxy.

The above process of tuning the miniature marker assembly 10 allows forextremely accurate tuning during mass production of the markerassemblies while minimizing the degree of variation between two markerassemblies tuned to the same frequency. The signal elements 18 of markerassemblies 10 having a tuning accuracy in the range of ±0.5% of thedesired resonant frequency can be economically manufactured in largequantities. In one embodiment, signal elements 18 can be economicallytuned consistently during manufacturing with a margin of error as low as±0.2% of the desired resonant frequency.

In another embodiment, the resonant frequency of the signal element 18can be finely tuned by removing material from the ferromagnetic core 14by, as an example, laser cutting or machining ferromagnetic materialfrom an endcap 32 or the core's central portion 28. In anotherembodiment, fine tuning of the signal element 18 can be accomplished byremoving material from the capacitor 16. The resonant frequency of thesignal element 18 can be actively or dynamically measured during thematerial removal process to determine when the phase shift is zero andinductive reactance equals the capacitive reactance of the signalelement.

After the endcap 32 is fixed in position on the core's central portion28, the tuned signal element 18 can be carefully slid into theencapsulation member 20 (FIG. 1). The encapsulation member 20 is sealedto provide a fully contained inert miniature marker assembly 10specifically tuned to a selected frequency. One of the fragile areas ofthe marker assembly 10 can be the connection between the capacitor 16and the wire leads from the coil 12. The connections can be susceptibleto damage during the manufacturing process because of the small diameterof the coil wire. Therefore, the signal element 18 must be carefullyinserted into the encapsulation member 20. In one embodiment, thecapacitor/coil interface is sealed with a flexible epoxy or otheradhesive that electrically insulates the lead wires. Sealing the leadwires also protects the lead wires from corrosion if any fluid from thepatient tissue contacts the signal element 18.

FIG. 6 is an exploded isometric view of a marker assembly 10 of analternate embodiment. FIG. 7 is a cross-sectional view takensubstantially along line 7-7 of FIG. 6 showing the inductor portion ofthe marker assembly 10 in an assembled condition. The marker assembly 10includes a plastic sleeve 40 on which the coil 12 is wound. In thisillustrated embodiment, the ferromagnetic core 14 is made of twoferromagnetic endcaps 42 and a separate ferromagnetic rod 44. Theferromagnetic endcaps 42 are positioned on the ends of the plasticsleeve 40, and the coil 12 is positioned between the endcaps. Theferromagnetic rod 44 is shaped and sized to slide into the plasticsleeve 40. During the manufacturing and tuning of the marker assembly10, the ferromagnetic rod 44 is axially movable in the sleeve relativeto the coil 12 and the ferromagnetic endcaps 42 to change the resonantfrequency.

In the illustrated embodiment, a preformed plastic sleeve 40 is used inthe coil winding process so the coil 12 is wound directly onto thesleeve in the tight winding configuration as discussed above. Theferromagnetic endcaps 42 can be adhered to the plastic sleeve 40 eitherbefore or after the winding process. After the coil 12 and theferromagnetic endcaps 42 are securely positioned on the plastic sleeve40, the ferromagnetic rod 44 is inserted into the plastic sleeve andaxially positioned relative to the coil 12 until the target resonantfrequency of the signal element 18 is achieved. In one embodiment, theresonant frequency is measured dynamically as the ferromagnetic rod 44is adjusted axially relative to the coil 12 until the inductivereactance matches the capacitive reactance. This dynamic measuring ofthe resonant frequency allows the ferromagnetic rod 44 to be preciselypositioned during tuning of the marker assembly 10. Accordingly, themarker assembly 10 provides the maximum tunability to a desiredinductance for a given frequency.

As best seen in FIG. 7, the ferromagnetic rod 44 is positioned in theplastic sleeve 40 coaxially with respect to the coil 12 such that thegeometric centers 41 and 43 of the rod and the coil, respectively, areoffset by a selected distance when a target resonant frequency isnominally achieved. The ferromagnetic rod 44 can then be moved axiallyin either direction to increase or decrease the resonant frequency toaccommodate the maximum variation possible in the coil 12 or thecapacitor 16 while still obtaining the target resonant frequency. Theaxial adjustments of the ferromagnetic rod 44 can be minimal because theinductance changes exponentially as a function of the distance betweenthe geometric center 43 of the coil 12 and the geometric center 41 ofthe ferromagnetic rod. Thus, axial movement of the ferromagnetic rod 44provides extremely fine tuning of the marker assembly's signal element18 with a minimum degree of axial movement.

Once the ferromagnetic rod 44 is positioned in the plastic sleeve 40 toachieve the specific inductance, thereby tuning the signal element 18 toresonate at a selected frequency, the ferromagnetic rod is securelyaffixed in position with an adhesive. In one embodiment, the adhesive isa UV-cured epoxy, a polyurethane adhesive, or a ferromagnetic-basedadhesive. In the case of the ferromagnetic-based adhesive, the uncuredadhesive should be present in the bond line joint during tuning so thatthe resonant frequency does not shift. The tuned signal element 18 isthen encapsulated in the inert encapsulation member 20 (FIG. 1) asdiscussed above.

In a simpler alternate configuration, ferromagnetic endcaps are notincorporated in the signal element 18 and the ferromagnetic core 14 maybe just the ferromagnetic rod 44 having a length the same as the coil12, shorter than the coil, or longer than the coil. The signal element18 is then tuned by moving the ferromagnetic rod 44 within the coil 12,which may or may not be wound on a plastic sleeve. Ferromagnetic pasteor adhesive may also be used in place of preformed rigid ferromagneticcores. In this configuration, the signal element is tuned to a specificresonant frequency by injecting ferromagnetic paste into the center ofthe coil.

The encapsulation member 20 can be a plastic or glass sleeve sealed onboth ends so as to fully enclose the tuned signal element 18. For markerassemblies 10 designed for use in medical procedures in which the markerassemblies may be permanently implanted in a patient, the signalelements 18 are encapsulated in part to protect the patient's tissuesfrom exposure to any non-biocompatible materials that may be used tooptimize the marker signals. The encapsulation member 20 also insulatesthe signal element 18 from bodily fluids that may cause corrosion oroxidation of the signal element and affect its performance. Theencapsulation member 20 hermetically seals the signal element 18 withoutadversely affecting the signal element or its emitted signal.

In one embodiment the signal element is encapsulated in a strong, rigid,and biocompatible material such as epoxy that helps protect the signalelement 18 from mechanical damage.

Alternatively, the signal element 18 can be placed inside of abiocompatible encapsulation sleeve 49 as shown in FIG. 8. Theencapsulation sleeve 49 and signal element 18 form an assembly 51 thatcan be completely encased in a biocompatible material, such as UV-curedor heat-cured epoxy 53. Alternatively, rather than potting the entireassembly 51, another encapsulating material, such as epoxy, could beinjected inside the encapsulation sleeve 49 to encase the signal element18 so that the outer wall of the plastic or glass encapsulation sleeveis in direct contact with the tissue. Materials that are appropriate forplastic encapsulation sleeves 49 can include, as an example, polyamide,polyetherblockamide, polyimide, polyurethane, polyetheretherketone.Biocompatible glass may also be an acceptable material for encapsulationsleeves 49.

Some benefits of using an encapsulation sleeve 49 are that the sleevecan be used to help center the signal element 18, help to prevent damageto the signal element during handling, and help make the encapsulationprocess easier by preventing the signal element from being exposedduring encapsulation. For example, if the signal element 18 was notplaced in an encapsulation sleeve 49 prior to potting with epoxy, thesignal element may move to the side of the mold and be exposed after theepoxy had cured. To prevent this phenomena from occurring, the signalelement 18 needs to be placed in a sleeve 49 or otherwise coated beforecompleting the final encapsulation process.

In another embodiment, the encapsulation member 20 is composed ofsturdy, durable, biocompatible heat-shrink tubing. The signal element 18is placed in the heat-shrink material, and heat is applied until thematerial shrinks and encases the signal element. The heat-shrinkmaterial is selected so that the heat necessary to shrink the materialis not high enough to damage the signal element 18. In an additionalembodiment, the signal element 18 and the shrunken encapsulating sleeveare potted in epoxy to fully enclose and seal the signal element 18.

The signal element 18 in other embodiments can be encapsulated with oneor more materials. Using multiple materials can make the encapsulationprocess easier and can allow additional functionality to be incorporatedinto the marker assembly 10. As an example, the signal element 18 can beencapsulated with a rigid biocompatible material, such as an epoxy, toprotect the components from damage. The epoxy also would provide a rigidbase for, as an example, attaching a fastener for mechanically attachingthe marker assembly to tissue in the patient, thereby preventing themarker assembly from migrating immediately following marker deploymentin the patient and during the implant duration.

An additional biocompatible coating material may be placed on theoutside of the rigid encapsulation member 20 to react with the tissueand help the marker assembly 10 adhere to the tissue. The additionalcoating material can also help promote or accelerate tissueencapsulation of the marker. Eliciting a specific biological response atthe marker/tissue interface could also be beneficial in preventingmarker assembly migration, especially in soft tissue.

In another embodiment, the encapsulation member 20 includes abiocompatible, thin-walled glass or plastic vial. These vials aresimilar to the previously described encapsulation sleeves 49 except oneend is already sealed prior to insertion of the signal element 18. Thesignal element 18 is placed into the vial and an encapsulation material,such as a quick-curing adhesive or epoxy, is injected into the vial tofully encase the signal element. After the signal element 18 is placedin the vial, the epoxy or other material is dispensed into the vial soas to avoid entrapping air bubbles in the vial. Avoiding air bubbles isimportant because if the vial is damaged at the site of a large bubble,part of the signal element may be exposed to body fluids and tissues.When the signal element 18 is placed in the previously discussed glassor plastic small-bore sleeve, epoxy can be injected into the sleeve tomore easily avoid the formation of air bubbles within the sleeve.

Using glass can be advantageous because glass is inert and insoluble.The characteristics of glass are also very desirable for long-termimplantation of marker assemblies. If undamaged, the glass will preventleakage (egress) of incompatible materials from the signal element 18and ingress of tissue fluids to the signal element. In anotherembodiment, powdered or granular glass is combined with biocompatibleepoxy and used to encapsulate the signal elements 18 placed in sleevesor vials. In another embodiment the signal element 18 can be securedwith an adhesive inside an encapsulation sleeve or vial and the end(s)of the sleeve/vial can be closed using heat or a laser to melt and sealthe end(s) of the vial or sleeve.

FIG. 8 is a cross-sectional view of a marker assembly 10 of an alternateembodiment. The marker assembly 10 has the plastic sleeve 40 aroundwhich the coil 12 is wound. The ferromagnetic core 14 is a two-piececore, with each core piece 48 having an endcap 32 and a first section ofthe core's central portion 28 integrally connected to each other. Eachcore piece 48 is positioned so that the section of the core's centralportion 28 extends into the plastic sleeve 40 until the endcap 32 isadjacent to the coil 12. The two core pieces 48 are coaxially alignedand immediately adjacent to each other within the plastic sleeve 40.

Tuning of the marker assembly's 10 signal element 18 is achieved byaxially moving one or both of the core pieces 48 relative to each otherand to the coil 12. The resonant frequency can be actively measuredduring the axial movement of one or both of the core pieces 48 until theinductive reactance of the signal element 18 and the capacitivereactance are matched for the selected frequency. After the markerassembly's signal element 18 has been accurately tuned to resonate atthe selected frequency, the core pieces 48 are fixed in positionrelative to the plastic sleeve 40 and the coil 12 by an adhesive. Thesignal element 18 can be inserted into an encapsulation member andhermetically sealed to provide the fully enclosed and inert miniaturemarker assembly 10 suitable for implantation in a patient.

FIG. 9 is an isometric view of a marker assembly 10 in accordance withanother alternate embodiment. In this embodiment, the marker assembly 10includes the capacitor 16, the coil 12 connected to the capacitor, andthe ferromagnetic core 14. The core 14 includes a ferromagnetic rod 44extending through the coil 12, and endcaps 32 are attached to theferromagnetic rod. In FIG. 9, an endcap 32 on the signal elementopposite the capacitor 16 and the encapsulation member 20 are not shownfor illustrative purposes. The coil 12 in this embodiment is made froman elongated wire having an outer airbondable coating that is on top ofthe wire insulation. Prior to winding the wire around, as an example, amanufacturing machine's mandrel, the airbondable wire is heated to makethe coating sufficiently tacky so that the wire adheres to adjacentwindings as it is wound to form the coil 12. After the coil 12 has beenwound, the airbondable coating has set, and the coil has been removedfrom the manufacturing machine's mandrel, the coil is a freestandingcoil with an open interior area 50.

In the illustrated embodiment, a coil winding machine is used to windthe coil 12 onto a metal mandrel and then eject the coil off themandrel. Winding the coil 12 onto a metal mandrel has benefits overwinding directly onto the small diameter (e.g., 0.75 mm) ferromagneticrod 44 because the ferromagnetic rod must be supported on both ends toprevent fracture and/or breakage. Securely holding the ferromagnetic rod44 with a spindle and tail stock of the winding machine requiresconcentricity of the small components, because slight misalignment canalso lead to fracture and/or breakage of the ferromagnetic rod. Windingthe coil 12 onto a metal mandrel of a coil winding device rather thandirectly onto the ferromagnetic core 44 is more conducive to a fullyautomated process because the small ferromagnetic core does not have tobe preloaded prior to starting the winding process.

The core 14 is positioned so the ferromagnetic rod 44 extends through aninterior area 50 in the coil 12. The coil's interior area 50 is sized toprovide a clearance fit with the ferromagnetic rod 44, so there is aslight space between the ferromagnetic rod and the inner wall of thecoil 12. A ferromagnetic-based adhesive 52 can be used at the interfacebetween the ferromagnetic rod 44 and the coil 12 during assembly toeliminate the air gap between the components, to provide additionalferromagnetic material within the coil, and to secure the ferromagneticrod 44 to the coil 12. Adding additional ferromagnetic increases theinductance of the signal element 18 without increasing the signalelement's size. The endcaps 32 are attached to the ends of theferromagnetic rod 44 with the coil 12 between them.

The signal element 18 can be tuned to the selected resonant frequency byaxially adjusting the ferromagnetic rod 44 and/or an endcap 32 relativeto the coil 12. Once the signal element 18 is precisely tuned, theadhesive 52 is allowed to set or cure, and the ferromagnetic rod 44 isfixed in position in the coil 12 and the endcap 32 is fixed in positionon the ferromagnetic rod. The tuned signal element 18 can then bepositioned in an encapsulating member 20 (FIG. 1) to provide the inertminiature resonating marker assembly 10.

FIG. 10 is an isometric view of an alternate embodiment of aferromagnetic core 60 of a marker assembly 10. FIG. 11 is an enlargedisometric view of a recessed endcap 66 of the core 60 of FIG. 10. Thecore 60 (FIG. 10) of the illustrated embodiment has an enlarged,substantially solid endcap 62 connected to one end of a smaller diameterferromagnetic rod 64, and a second endcap 66 is attached to the oppositeend of the ferromagnetic rod. The second endcap 66 has a recess 68shaped and sized to receive a capacitor 16 (FIG. 11) therein. Thecapacitor 16 in the illustrated embodiment is securely retained in placewithin the recess 68 by a non-conductive adhesive. The coil 12 is woundaround the ferromagnetic rod 64 and operatively connected to theterminals 73 of the capacitor 16 so as to form the signal element 18.

The recess 68 in the endcap 66 can be formed by grinding, molding, orlaser cutting the endcap. The recess 68 is designed to position thecapacitor 16 in a selected orientation relative to the coil 12 suchthat, when the coil is installed or wound onto the ferromagnetic rod 64,a minimum length of lead wire 75 is needed to connect the coil to thecapacitor. This orientation of the recess 68 and the capacitor 16 alsoprovides a compact tunable configuration that increases commonality ofthe miniature marker assembly 10, which makes mass production of tunedmarker assemblies 10 more efficient and economical.

In one embodiment, a spacer or coating is provided within the recess 68between the endcap 66 and the capacitor 16. The spacer or coating canhelp avoid electrical shorts with the capacitor 16 and can provide asurface with different bonding characteristics to hold the capacitor inplace.

The small physical size and fragility of the signal element 18 are suchthat complete assembly of the tuned signal element can be difficult. Thesmall capacitors 16, especially the standard 0603, 0504, and 0402 sizecapacitors, can be dramatically affected during the process of solderingthe lead wires 75 to the terminals 73. These small capacitors 16 havesuch high ratios of contact surface area to total volume that they caninadvertently adhere to a soldering tip during the assembly process,which can result in a failed joint or a damaged capacitor.

Mounting the capacitor 16 in the recess 68 helps alleviate themanufacturing difficulties, because the endcap 66 holds the capacitor inplace with the proper, known, and consistent orientation with respect tothe coil 12 and lead wires 75. When the capacitor 16 is centered in therecess 68, the capacitor is captured between the side portions of theendcap 66. The side portions work with the adhesive to provide aprotective structural support for the capacitor 16. With this method, asimple axial load can be applied to retain the capacitor 16 within therecess 68 during application and curing of an adhesive. Once thecapacitor 16 is structurally secured in the recessed endcap 66, the coillead wires 75 may be readily soldered to the capacitor terminals 73.

As seen in FIG. 12, in one alternate embodiment, a flat section 72 isformed on the side of the endcap 66 adjacent to the recess 68 toaccommodate the coil lead wires 75 (FIG. 11). The flat sections 72 helpprevent the coil 12's fragile lead wires 75 from being snagged ordamaged during the process of encasing the completed signal element 18into the encapsulation member 20 (FIG. 1). In this alternate embodiment,the endcap 66 is also provided with an aperture 77 shaped and sized toreceive the core's ferromagnetic rod 64 (FIG. 10) so that the endcap maybe axially adjusted on the ferromagnetic rod to tune the signal element18. The other endcap 62 (FIG. 10) without the recess can also be axiallyadjustable on the ferromagnetic rod 64 to tune the signal element 18 asdiscussed above.

In an alternate configuration, the connections between the coil 12 andthe capacitor 16 are covered or sealed with an adhesive or epoxy priorto the encapsulation process. Sealing the connections helps preventdamage during the encapsulation process and helps prevent corrosion ofthe signal element in cases where the marker encapsulation is damagedduring or prior to implantation. The connection between the coil 12 andcapacitor 16 is an area of the signal element where the small diameterwire is not protected by a coating of insulation.

As best seen in FIG. 13, another embodiment of the recessed endcap 66includes a groove 74 formed in the outer surface of the endcap adjacentto the recess 68. The groove 74 is sized and positioned to accommodatethe coil 12's lead wires 75 (FIG. 11). The groove 74 helps to protectthe lead wires 75 from being damaged during the encapsulation process ofthe signal element 18 to form a hermetically sealed, miniatureresonating marker assembly 10.

FIG. 14 is a cross-sectional view of a miniature resonating markerassembly 10 of an alternate embodiment In this alternate embodiment, themarker assembly 10 includes a signal element 80 having a ferromagneticcore 82, a wire coil 84, and an annular-shaped capacitor 86 to form thesignal element that resonates at a selected frequency upon externalexcitation. The ferromagnetic core 82 in the illustrated embodimentincludes a central ferromagnetic rod 88 and a pair of enlarged endcaps90 secured to the ends of the ferromagnetic rod. The endcaps 90 aredisk-shaped members with a central aperture that receives theferromagnetic rod 88 so the endcaps can be axially adjusted to tune thesignal element 80 before the endcaps are fixed in place. In an alternateembodiment, one or both of the endcaps 90 can be integrally connected tothe ferromagnetic rod 88.

The annular-shaped capacitor 86 in this alternate embodiment has asimilar disk shape and outer diameter as that of the endcaps 90. Acentral aperture 92 extends through the capacitor 86, and theferromagnetic rod 88 extends through the capacitor's central aperture.Accordingly, the capacitor 86 is positioned generally in the middle ofthe signal element 80 between the endcaps 90.

The coil 84 is wound in two segments onto the ferromagnetic rod 88between the endcaps 90 and the capacitor 86. Accordingly, one coilsegment 93 is wound around the ferromagnetic rod 88 between one endcap90 and the capacitor 86, and a second coil segment 95 is wound aroundthe ferromagnetic rod between the other endcap and the other side of thecapacitor. In one embodiment, the two coil segments 93 and 95 are formedby one continuous wire. In an alternate embodiment, the coil segments 93and 95 may be formed by separate wires interconnected to provide theelectrical continuity of the coil 84.

The signal element 80 of this marker assembly 10 is tuned in oneembodiment by axially adjusting the position of one or both of theendcaps 90 relative to the ferromagnetic rod 88 and the coil 84 untilthe signal element is tuned to resonate at the selected frequency. Oncethe signal element 80 is tuned and the endcaps 90 are secured inposition on the ferromagnetic rod 88, the signal element can beencapsulated to provide the inert resonating marker assembly 10.

FIG. 15 is an isometric view showing an alternate embodiment of aminiature resonating marker assembly 96. FIG. 16 is a cross-sectionalview of the marker assembly 96 of FIG. 15. The marker assembly 96includes a ferromagnetic core 100 with an I-beam shaped cross-section.The core 100 has an elongated web 102 extending between a pair offerromagnetic flanges 104 that act as endcaps. In this embodiment, theflanges 104 and the web 102 are integrally connected. In alternateembodiments, the I-beam configuration can be formed by separate flanges104 adhered or otherwise suitably attached to the web 102.

The marker assembly 96 also includes an elongated coil 106 tightly woundaround the web 102 between the flanges 104. The I-beam configuration ofthe core 100 allows each winding of the elongated coil 106 to cover moreof the core 100 than the previously described marker configurations,thereby allowing for more inductance with less wire. In one embodiment,the web 102 has a shorter axial length than the flanges 104, such thatthe end portions of the web are recessed. The coil 106 is wound aroundthe recessed web 102 so the coil does not extend beyond the ends of theflanges 104 to increase the length of the marker assembly 96. Acapacitor 108 (FIG. 15) is securely mounted to one end of theferromagnetic core 100 and is operatively connected to the lead wiresfrom the coil 106. The capacitor 108 is, thus, substantially coaxiallyaligned with the core 100 and the coil 106 so as to provide an elongatedsignal element 98 insertable into an encapsulating member 107 and sealedto provide the inert, miniature marker assembly 96.

FIG. 17 is a cross-sectional view of an alternate embodiment of themarker assembly 96. This alternate marker assembly 96 includes anelongated ferromagnetic core 110 with a central web 112 and a pair ofendcaps 114 connected to the web. Each endcap 114 has a curved orarcuate upper surface 116 that provides for a smooth, rounded signalelement 98 that can be easily and quickly inserted into a generallycylindrical encapsulation member 117. Accordingly, the shape of thesignal element 98 helps avoid misalignment or binding with theencapsulation member 117 during the installation process in the smallencapsulation member. While the illustrated embodiments show geometricconfigurations of I-beam shapes with flat or curved flanges, othergeometric configurations can be used to form the endcaps while providinga suitable area to retain the windings of the coil 118 therebetween.

FIG. 18 is a cross-sectional view of a modular miniature marker assembly130 of an alternate embodiment. The marker assembly 130 includes a tunedsignal element 132 encapsulated in an outer, inert encapsulation member134 sealed at its ends to fully enclose the signal element. The signalelement 132 in the illustrated embodiment includes a ferromagnetic core136, a coil 138 wound about the core in a tight-winding configurationdiscussed above and shown in FIG. 3. A capacitor 140 is attached to thecore 136 and operatively connected to the coil 138. The coil 138 and thecapacitor 140 are similar to those described above in the embodiments.The core 136 includes a central ferromagnetic rod 142 that extendsaxially through the coil 138, and a pair of modular endcaps 144 aremounted on the ferromagnetic rod on opposite sides of the coil 138.

The modular endcaps 144 are made up of a plurality of annular-shapedferromagnetic disks 148 each with a central aperture 149 that receivesthe end portion of the ferromagnetic rod 142. If an asymmetric core 136is desired, the modular endcaps 144 can be easily constructed with adifferent number of ferromagnetic disks 148 or with disks of differentthicknesses, so as to achieve the selected ultimate volume offerromagnetic material at each end of the ferromagnetic rod 142.Accordingly, the characteristics of the signal element 132 can beselected and/or modified by combining a different number or type offerromagnetic disks 148 to form the modular endcaps 144.

When a large number of miniature marker assemblies 130 are to bemanufactured, the same modular components of the ferromagnetic core 136can be used in different combinations to achieve the tuned markerassembly. Once the modular endcaps 144 are in place on the ferromagneticrod 142, the ferromagnetic disks 148 can be adjusted axially relative tothe ferromagnetic rod and the coil 138 to actively tune the markerassembly 130 to resonate at the selected frequency. Alternatively, theferromagnetic rod 142 may be axially moved relative to the coil 132 andthe modular endcaps 144 to achieve the necessary inductance to tune thesignal element 132. After the signal element 132 is accurately tuned,the modular endcaps 144 and central ferromagnetic rod 142 are fixed inposition so the signal element remains accurately tuned. The signalelement 132 can then be inserted into the encapsulation member 134 andhermetically sealed.

In an alternate embodiment, the characteristics of the signal element132 may be controlled by combining a central rod 142 made of oneferromagnetic material, and enlarged endcaps 144 made of anotherferromagnetic material. The ferromagnetic material for the rod 142 has afirst magnetic permeability and saturates at a high field strength, andthe ferromagnetic material for the endcaps 144 has a greater magneticpermeability and saturates at a lower field strength. Accordingly, theferromagnetic core 136 can have a substantially uniform saturationcharacteristic throughout the core, thereby minimizing the effects ofsaturation on the resonating marker assembly 130. As an example, in oneembodiment, the endcaps 144 may be made of Fair Rite 78 and theferromagnetic rod 142 may be manufactured of a power ferromagnetichaving less magnetic permeability and a higher saturation level. Theferromagnetic rod 142 made of the power ferromagnetic material allowsmore windings around the rod to form the coil 138 without saturation ofthe central ferromagnetic rod.

The windings and the core 136 including endcaps 144 of the signalelement 132 makeup the elements of the signal element's inductor. For afirst order approximation, the inductance of the signal element 132 isoptimized for a given volume when the number of turns is maximized, theenclosed area for each turn is maximized, the length of the coil isminimized, the initial permeability of the core is maximized, and thelength/diameter ratio of the core is maximized. Also, for a first orderapproximation, the quality factor (Q factor) of the signal element 132is optimized when the inductance is maximized and the resistance of thewindings and core 136 is minimized. Typically, the resistance of thewindings and core 136 will be nearly equal at the chosen frequency whenthe inductor has been optimized. Other items that must be addressed whenoptimizing the Q factor include multiple layer windings, the presence ofendcaps 144, and frequency dependent losses including dielectric lossand ferrite loss. Accordingly, the marker assembly 130 is designed toprovide a maximum Q factor for the marker assembly at a chosenfrequency. The embodiment utilizing endcaps 144 with a material having adifferent permeability than the ferromagnetic rod 142 can be eithermodular endcaps as shown in FIG. 18, or can be single, solid endcaps asdiscussed above and shown, for example, in FIG. 2. Accordingly, theminiature resonating marker assembly 130 can be manufactured withcomponents of different materials and characteristics to provide extremeversatility for desired marker characteristics while allowing the markerassemblies to be manufactured efficiently and cost effectively.

The process of manufacturing the miniature marker assembly 10 in oneembodiment involves actively measuring the marker assembly's inductanceduring the coil winding process. The process is discussed with referenceto FIG. 19. When producing a tuned signal element, which generallyconsists of an inductor (L) and a capacitor (C) interconnected either inseries or in parallel, one significant difficulty is producing aninductor of precise inductance to match imprecise capacitors havingtolerances of ±5% or more. Such capacitors are readily available from anumber of manufacturers. The manufacturing process of dynamicallymeasuring the inductance of miniature resonating marker coils 154 allowsprecise control of the inductance of very small inductors, (e.g., withlengths in the range of approximately 1-13 mm, inclusive).

In one embodiment of the manufacturing method, a core 160 with acapacitor 162 affixed thereto is installed in a coil winding machine166. After the core 160 is positioned in the winding machine 166, a testprobe advances forward and makes contact with the capacitor 162 at onetest point, such as the capacitor terminal 168. A second test probe isadvanced forward to make contact with the other capacitor contact. Thecapacitor's capacitance is measured and the test probes are retracted.From the capacitance, an empirically derived formula based on the chosenwire size and the core material and other critical inductor parametersare used to identify the inductance value necessary to obtain thedesired resonant frequency for the LC series circuit being produced. Asingle, long strand of wire 164 is connected at one end to the capacitorterminal 168 on the capacitor 162. The coil winding machine 166 isactivated to spin the core 160 and capacitor 162 as a unit so as totightly wind the wire 164 onto the central rod 150 of the ferromagneticcore between the endcaps 152. The windings are controlled to beimmediately adjacent to each other so as to minimize the amount of spacebetween windings, as discussed above and shown in FIG. 3.

The individual characteristics of the capacitor 162, the ferromagneticcore 160, and the wire 164 are generally known, so the approximatenumber of windings needed in the coil 154 can be calculated toapproximately achieve the desired inductance needed to tune the signalelement. The coil winding machine 166 is used to wind the coil 154 withthe initially calculated number of windings. In one embodiment, the coil154 is wound about the core 160 with a selected number of turnscalculated with the empirically derived formula to achieve an inductancevalue slightly less than necessary to obtain the desired LC seriescircuit resonant frequency.

After the calculated number of windings are wound onto the core 160, atest probe advances forward making contact with one of the capacitorterminals 168 to which the wire is connected, and another test probe isadvanced to make contact with the other end of the wire 164 at the wiretakeoff spool (not shown) on the coil winding machine 166. Theinductance is dynamically measured and compared to the actual inductancerequired to obtain the desired resonant frequency of the LC seriescircuit. The difference between the actual inductance and the targetinductance is used to calculate the additional number of windingsnecessary to add to the coil 154 to accurately achieve the targetinductance within a very small margin of error during the manufacturingprocess. The calculated number of additional windings are added to thecoil 154 to achieve the target inductance.

In one embodiment, the actual inductance is measured again after theadditional windings are added to the coil 154 to confirm that the actualinductance matches the target inductance within a selected margin oferror, as an example, in the range of approximately ±0.2% of the targetinductance. The dynamic inductance measurement process can beiteratively used as many times as needed to achieve the inductanceneeded to obtain the fine-tuned LC series circuit. In one embodiment, ifthe measured inductance is within 0.5% of the required tuned circuitinductance, the additional windings are added one at a time, with adynamic inductance measurement taken after each winding is added untilthe measured inductance matches the required tuned circuit inductancewithin the resolution of the inductance measuring device.

In one embodiment wherein the coil 154 is wound onto a dumbbell-shapedferromagnetic core 160, an initial manual process begins with a startturn termination. A small bead of adhesive (e.g., UV-spot-curing type orthe like) is used to secure the end of the wire 164 to the ferromagneticcore 160 immediately adjacent to the winding area on the circumferenceof one of the endcaps 152. The adhesive also serves as a strain relieffor the eventual termination to the capacitor terminal 168. The exposedlength of wire 164 is twisted back on itself and tinned to provide oneof the electrical connections for the dynamic inductance measurement.Following the winding of the first iteration of windings, a service loopis drawn from the wire spool from a location between the spool and theinput to a tensioner device. The length of the service loop isdetermined by the empirically derived formula. As with the start turntermination, the end of the uncut service loop is twisted back on itselfand tinned. This provides a second electrical connection for the dynamicinductance measurement.

In the illustrated embodiment, the method of dynamic tuning of the LCseries circuit for the resonating marker assembly 10 is used forautomated manufacturing of the miniature resonating marker assembliestuned within a tolerance of approximately ±0.5% of the target resonantfrequency. Such highly accurate tuning of the resonating marker assembly10 allows for extremely accurate manufacturing of relatively largevolumes of the miniature resonating marker assemblies in a veryefficient and cost-effective manner. In addition, large quantities ofminiature resonating marker assemblies 10 having different resonatingfrequencies can be manufactured efficiently and cost effectively.

Although specific embodiments of, and examples for, the presentinvention are described herein for illustrative purposes, variousequivalent modifications can be made without departing from the spiritand scope of the invention, as will be recognized by those skilled inthe relevant art. The teachings provided herein of the present inventioncan be applied to resonating marker assemblies, not necessarily theexemplary implantable resonating marker assemblies generally describedabove.

These and other changes can be made to the invention in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the invention to thespecific embodiments disclosed in the specification and the claims, butshould be construed to include all miniature resonating markerassemblies that operate in accordance with the claims. Accordingly, theinvention is not limited by the disclosure, but instead its scope is tobe determined entirely by the following claims.

1. A method of actively tuning a resonating marker assembly to have aselected resonant frequency value, comprising: winding an elongated wirearound a central portion of a ferromagnetic core intermediate a pair offerromagnetic endcaps of the core to form a coil with a plurality ofwindings, the coil and core forming a combination with a firstinductance value; measuring the first inductance value of thecombination; comparing the first inductance to the selected inductancevalue; and adjusting the amount of wire forming the coil after comparingthe first inductance value to the selected inductance value by adding orremoving one or more turns from the coil until the inductance value ofthe combination is substantially equal to the selected inductance value.2. A method of tuning a miniature resonating marker assembly to aselected resonant frequency, comprising: placing a ferromagnetic corewithin a wire coil having a plurality of windings to form an inductor;connecting lead wires of the inductor to a capacitor, the capacitorbeing adjacent to the core to form a miniature signal element; excitingthe marker assembly at a known frequency; and measuring the markersignal intensity or signal phase at the frequency of interest; andadjusting the core axially relative to the windings to adjust the actualinductance until the resonant frequency of the marker matches thedesired frequency.
 3. A method of tuning a miniature resonating markerassembly to a selected resonant frequency where the impedance of theinductor and capacitor are matched at a selected resonant frequency,comprising: placing a core within a wire coil having a plurality ofwindings; connecting lead wires of the inductor to a capacitor with theknown capacitance, the capacitor being adjacent to the core to form aminiature signal element; measuring the actual resonant frequency of thesignal element; comparing the actual resonant frequency of the signalelement to the selected resonant frequency; and adjusting the coreaxially relative to the winding to adjust the actual resonant frequencyuntil the actual resonant frequency is substantially equal to theselected resonant frequency.
 4. The method of claim 3, furthercomprising fixing the core to the winding to prevent axial movementtherebetween after the actual resonant frequency is substantially equalto the selected resonant frequency.
 5. The method of claim 3, furthercomprising winding the coil onto a sleeve with an interior area, andplacing the core within the wire coil including placing the core withinthe interior area of the sleeve.
 6. The method of claim 3, furthercomprising encapsulating the core, the coil, and the capacitor in aninert encapsulation member, and sealing the encapsulation member tohermetically contain the tuned marker assembly.
 7. The method of claim 3wherein the core includes a central portion within the coil and a pairof endcaps, and further comprising positioning an endcap on an endportion of the core's central portion adjacent to the coil, andadjusting the core axially including moving one of the endcaps axiallyon the central portion relative to the coil.
 8. The method of claim 7,further comprising securing the endcap on the core's central portion ina fixed location after the resonant frequency is substantially equal tothe selected resonant frequency.
 9. The method of claim 3, furthercomprising forming the wire coil from a plurality of turns of a wirewith an airbondable coating thereon to adhere each turn to one or moreadjacent turns forming a self supporting winding.
 10. The method ofclaim 3, further comprising securing the core in a fixed positionrelative to the winding with a ferromagnetic -based adhesive when theactual resonant frequency is substantially equal to the selectedresonant frequency.
 11. A method of making a tuned, miniature resonatingmarker assembly with a selected inductance, a known capacitance, and aselected resonant frequency comprising: placing a wire coil around aferromagnetic core having a central portion within the coil and a pairof ferromagnetic endcaps attached to the central portion adjacent to thecoil; connecting a capacitor to lead wires of the wire coil, thecapacitor having a known capacitance and being adjacent to the core, thecoil, and the capacitor to form an activatable assembly; measuring anactual impedance and phase of the activatable assembly; comparing anactual resonant frequency to the selected resonant frequency; andremoving ferromagnetic-material from the core to adjust the actualresonant frequency of the activatable assembly until the actual resonantfrequency is substantially equal to the selected resonant frequency. 12.A method of tuning a miniature resonating marker assembly to a selectedresonant frequency, comprising: placing a ferromagnetic core within awire coil having a plurality of windings to form an inductor; connectinglead wires of the inductor to a capacitor, the capacitor being adjacentto the core to form a miniature signal element; exciting the markerassembly at a known frequency; measuring the marker signal intensity orsignal phase at the frequency of interest; and removing ferromagneticmaterial from the core to adjust the actual inductance of theactivatable assembly until the resonant frequency of the marker matchesthe desired frequency.